cold water surge in re-circ system

I replaced a re-circ line for a client, water heater and added uponors demand pump with motion sensors. While showering about five minuits into and at approx recurring intervals the water is not going cold but just going to lukewarm for about 5 seconds and then returning to normal temps. this happens in both showers in the house older non pressure balanced valves. I replaced the line with pex could have the velocity increased due to the smaller inside diameter of the pex and could this be effecting the older valves? Any other thoiughts
thanks

What is the time interval on the motion sensors?

Once water reaches temp aquastat shuts pump off. does not allow motion to turn it back on untill after the water at the aquastat cools off. I replaced the pump once thinking the intragal check valve was bad, uponor was great to deal with but it did the same thing
thanks